Direct current signal receiver



Nov. 11, 1969 E. HERTER 3,478,175

DIRECT CURRENT SIGNALRECEIVER Filed Nov. 5, 1966 c1 ML I fl/ f Fig.3

United States Patent Int. Cl. no4m 3/20 7 US. Cl. 179-18 7 ClaimsABSTRACT OF THE DISCLOSURE A stable control AC voltage is coupled intothe control circuit of a transistor switching stage which controls thepower to a succeeding relay. The transistor is subject to control from aDCsource and in its quiescent state cannot be caused to operate by theAC voltage alone. Similarly, when operating'fully in accordance withdesign, the AC voltage alone will not interfere with continued operationof the transistor. If unusual interference signals are encountered,however, the AC signal will prevent power excursions which could damagethe transistor and also will produce anomalous outputs from thetransistor which can be used to signal failure of the system.

' The invention relates to a circuit arrangement to receive DC signalsand more particularly to such receivers comprising transistor controlledrelays wherein the operating curve of the transistor switching stageintersects the admissible power hyperbola.

When designing such receivers, the transistor is operated as a switch sothat the transistor loss is kept small. A relay of relatively high powercan be switched through such a transistor switching stage. Care must betaken, of course, when designing such a switching stage that, in thenon-operative condition, the transistor is safely blocked by asufficient bias and that, in the operative condition, the controlvoltage is sufficient to safely saturate the transistor. The operatingline or characteristic of the transistorized switching circuit canintersect the admissible power hyperbola, because this criticaloperating range is passed through with sufficient speed to precludedamaging the transistor.

If such a receiving device is operated by a DC fed line, the conditionsat open and closed line loop can be controlled without difficulties andthe relay can be switched as required. However, interferences of anykind, for example, leakages, short-circuits andexternal noise signals orvoltages, may cause the control voltage in the control circuit of thetransistor switching stage to drop until the transistor is operated atan inappropriate or wrong operating or Q-point. Further, theinterference can prevent the transistor switching stage from switchingso that the wrong operating condition prevails for a long time andleads, in case of an excessive load, frequently to the destruction ofthe transistor.

It is the object of the invention to improve the receiving circuitarrangement when the arrangement includes a transistor switching stagecontrolling a relay. The load line of the relay loaded transistorintersects the admissible power hyperbola so that even when thetransistor switching stage is improperly operated due to a faultcondition, the transistor itself is not destroyed and a signal toindicate the faulty condition can be derived, if so required.

The circuit arrangement to receive DC signals in accordance with theinvention, is provided with a regulated AC voltage that is coupled intothe control circuit of the transistor switching stage. When a DCoperating point is established the power loss of the transistor isperiodically changed with the AC voltage according to the powerhyperbola, so that the resulting power loss remains below the admissiblevalue of the constant power loss. The inventive arrangement prevents theproper switching operation from being adversely influenced. If thetransistor switching stage is not operated, the regulated or control ACvoltage is suppressed by a suitable bias in the control circuit and ifthe transistor switching stage is operated, the effect of the regulatedor control AC voltage in the output circuit is prevented by over-ridingof the control circuit.

The arrangement according to a further embodiment of the invention isstill more simplified in that the constant control AC voltage is commonfor several switching stages. It is quite possible thereby to providedifferent threshold voltages for the different switching stages. The.voltage dividers for these threshold voltages of the different switchingstages are mutually coupled via a common control AC voltage source.

In order to eliminate the reaction of the control AC voltage on thesupervised line, it is provided,according to a further embodiment of thearrangement, that, in case of a multi-stage arrangement of the receivingfacility, the constant AC control voltage is fed in behind the firstswitching stage or amplifying stage.

When operating single stage receiving devices the reaction of thecontrol AC voltage on the supervised line can be kept low by a voltageof low frequency. These interfering voltages on the line are notperceived, because sound effects caused by voltages of low frequency canhardly be heard.

If the receiving device is connected to a subscriber line or to ajunction line of a telephone exchange system, the constant control ACvoltage may be derived from the ringing AC voltage of the system. Inorder to keep the harmonies of this ringing voltage from the lines, asuitable filter, in the most simple case a leakage capacitor, is provided in the feeder circuit of the ringing AC voltage.

In case of faulty operation of a switching stage the controlled relayresponds and drops in the rhythm of the control AC voltage. Thisconstant change of the switching condition of the relay can be used in away known per se to indicate the faulty condition.

The invention is in detail explained with the aid of the accompanyingdrawings, wherein:

FIG. 1 shows an arrangement to receive DC signals, according to theinvention;

FIG. 2 shows a twin-stage transistor Witching stage according to theinvention; and

FIG. 3 shows the derivation of the common control AC voltage forswitching stages with different threshold voltages.

FIG. 1 shows two feeder resistor R of a DC-fed line a, b. The controlcircuit of the switching transistor Tr is coupled to the b-wire behindthe feeder resistor R via the resistor R1. The resistor R1 leads to thebase of transistor Tr, and conducts ground potential when the line isopen. The emitter of transistor Tr is connected to the tap of thevoltage divider, consisting of the resistors R2 and R3. The pnp-typetransistor Tr is blocked by the voltage drop at the resistor R2. Thecontrol AC voltage v1, coupled into the control circuit, increases thisblocking bias during one-half of the wave and reduces it in the otherhalf wave. The resulting votlage in the control circuit, however, alwaysis sutficient to block the transistor Tr so that it remainsnon-conductive. Therefore, relay A in the collector circuit of thetransistor is not energized when the line a, b is open.

When the line a, b is closed to a loop, the potential at the b-wirebecomes heavily negative so that the transistor Tr is actuated tosaturation. The bias across the voltage divider R2, R3 and the constantcontrol AC voltage v1 is insufficient to prevent the transistorsoperation at this control voltage, consequently, relay A is operated.These explanations show that a proper switching operation is obtaineddespite coupling in said control AC voltage.

However, if there is a fault causing the line to have a potentialapplied to it outside prescribed values, the transistor is operated viathe tapped control voltage to an operating point beyond the admissiblepower hyperbola, and then the control AC voltage v1 can be effective.This control AC voltage v1 can be selected so that the operating pointon the characteristic curve is periodically changed so that theresulting power loss drops below the admissible value of the constantpower loss hyperbola. The operation of the relay can thereby vary in therhythm of the control AC voltage between the operated and nonoperatedpositions. This constant response of relay A can be used in a simple wayto indicate trouble.

FIG. 1 shows that the control AC voltage v1 is derived for several suchswitching stages from a source such as the ringing AC voltage generatorG1. The arrow at the feeder winding of a transformer indicates thatseveral switching stages can be connected.

FIG. 2 shows a two-stage switching amplifier, the first stage of whichis coupled to the line to be supervised in the same way as shown inFIG. 1. The control circuit of transistor Tr is not coupled to the ACcontrol voltage, because this transistor does not have to deliver greatpower and can therefore be operated as an amplifier. The control ACvoltage v1 is coupled into the collector circuit of transistor Trthrough the resistor R4, so that for the following npn-type stagetransistor Tra the same operating conditions prevail as for thetransistor Tr shown in FIG. 1.

The operating bias for transistor Tra is obtained by the voltage sourceU/2 at the emitter of transistor Tra. The relay A in the collectorcircuit leads to ground potential.

It is also possible to supply several such two-stage switchingamplifiers by a common control AC voltage v1, as indicated by the arrowat the collector resistor R4.

FIG. 3 shows how different switching amplifiers can be power-fed by acommon control AC voltage v1. The voltage divider R2, R3 and R5, R6 andR7, R8 offer different biasses at their tapping points, which biassescan be led to several similar switching stages as shown in FIG. 1. Allvoltage dividers are led across the control AC voltage source v1, sothat this control AC voltage v1 is applied to all control circuits ofall switching stages.

The voltage v1 is derived from the ringing voltage from a generatorindicated at G1 through a transformer. As this ringing AC voltage showsa considerable percentage of harmonics, the secondary end of thetransformer is provided with a leakage capacitor C. It is therebyachieved that the interfering voltages reaching the lines over thecontrol are negligibly small.

While the principles of the invention have been described above inconnection with specific apparatus and applications, it is to beunderstood that this description is made only by way of example and notas a limitation on the scope of the invention.

I claim:

1. A direct current signalling receiver for receiving signalstransmitted over a line comprising a transistor forming a switchingstage, relay means controlled by said switching stage, means for biasingsaid switching stage to operate said transistor to a saturated conditionresponsive to a closed loop condition of said line and to operate saidtransistor to a non-saturated condition responsive to an open loopcondition of said line, said basing means providing an operatingcharacteristic for said switching stage that intersects the admissiblepower hyperbola, and means for coupling a constant alternating currentvoltage to the control circuit of said transistor, said alternatingcurrent voltage being selected to have insufficient magnitude to controlthe operation of the transistor in the presence of prescribed conditionsof operation of said loop but being selected to have sufficientmagnitude to control operation of said transistor when said loopoperation is outside said prescribed conditions.

2. The circuit arrangement of claim 1 wherein means are provided forcoupling said constant control AC voltage in common to several switchingstages.

3. The circuit arrangement of claim 2, including voltage dividers insaid bias circuits for providing threshold voltages for the differentswitching stages, means for connecting said threshold voltages over saidcommon, constant control AC voltage.

4. The circuit arrangement according to claim 3 wherein said switchingstage comprises a multi-stage arrangement, means for connecting theconstant control AC voltage to the first switching stage, and means inthe second switching stage for controlling the relay.

5. The circuit arrangement of claim 4 wherein means are provided forderiving the constant control AC voltage from ringing voltage.

6. The circuit arrangement of claim 5 wherein leakage capacitor meansare provided for filtering the harmonics of the ringing voltage.

7. The circuit arrangement of claim 4 wherein the relay is connected toa second switching stage, and in case of operation of the receivingdevice outside prescribed limits, the second switching stage causes therelay to respond and to drop in the rhythm of the contral AC voltage,and this constant switching of the relay is used to indicate the faultycondition.

References Cited UNITED STATES PATENTS 5/ 1943 Hall. 5/1958 Heetman.

